Such a telephone set is disclosed in, for example, Telesis Fall 1975, pp. 84-90.
When a telephone set comprises a monitoring amplifier, the loudspeaker being incorporated in the base of the set, oscillations may occur, so-called singing, due to acoustic feedback when the hand set is close to the base of the telephone set and the monitoring amplifier is on.
The acoustic coupling between the microphone and the loudspeaker could be compensated for by an attenuation network included in the loudspeaking channel and having a transfer characteristic which is the inverse of the acoustic transfer characteristic between the microphone and the loudspeaker. However, such a network will also affect normal speech transmission in the situations where there is no risk of oscillations due to acoustic feedback.